They have met on the biggest stages several times before. Even if Andy Murray hasn’t shown his top form since back surgery last September, he has saved his top form for Novak Djokovic.

A Murray-Djokovic quarterfinal is all set for Wednesday at the US Open.

Amid oppressive Flushing humidity, Djokovic and eighth-seeded Murray each posted straight-set, fourth-round victories Monday. The Brit did it inside Arthur Ashe Stadium over the power game of ninth-seeded Joe Wilfried Tsonga, 7-5, 7-5, 6-4 — Murray’s first win over a top 10-player in 2014.

Djokovic-Murray is a rematch of the 2013 Wimbledon final won by Murray, the 2012 Open final won by Murray and the Olympic semifinals held in London, also won by Murray.

“Very tough, tough draw,’’ Djokovic said. “I’m going to have to play my highest level in order to advance.’’

Indeed, back surgery or not, Djokovic knows he has a marathon battle ahead against Murray.

“He performs his best in the Grand Slams,’’ said Djokovic, coming off a Wimbleton title, marriage and announcement of a baby on the way. “Even though he had his back surgery last year that kept him off the tour a few months — he’s been on and off this year — but in the big matches he’s still fit. He still plays very high-quality tennis.’’

Djokovic has now advanced to 22 straight Grand Slam quarterfinals. In 2009, Djokovic lost in the third-round at the French Open to Kolschreiber. He has advanced to the fourth round and beyond in every Slam since.

“It says I do value these tournaments the most and try to always perform my best tennis in them,’’ Djokovic said. “Obviously it motivates me in the future to continue that streak.’’

Djokovic has won just one Open, and been the runner-up four times. Murray won his only Open over Djokovic in 2012 when he broke the British men’s Grand Slam hex in a five-hour, five-set match.

“We’ve had a lot of long ones,’’ Murray said. “We played a long one here a few years ago. I have great memories from that match. Hopefully we can play another top-level match. If I play well, I have a chance.’’

Murray says the long bouts versus Djokovic aren’t surprising because of their “similar style,’’ saying they “do a lot of the same things well.”

“Playing the No. 1 player in the world is exciting,’’ Murray said. “If you aren’t getting motivated and pumped for those matches, then that’s when there’s a problem and it’s time to maybe stop.’’

Murray, eliminated in the quarters of the Australian Open and Wimbledon, didn’t serve and volley but he posted 11 aces and strategically came to the net — winning 28-of-31 points there.

Murray said it was “a big concern’’ when he cramped up in his first-round match from drinking too much water. But he handled the heat perfectly Monday.

The always exciting Tsonga was up a break in the second and third sets but couldn’t sustain anything. Even the chair umpire couldn’t give him a break. In the second set, serving, Tsonga went through his serving motion but stopped when “a big bug came on my cap,’’ Tsonga said. It was ruled a fault.

This is the first Open Djokovic has had legend Boris Becker as a coach.

“People think he’s giving me advice to come to the net, actually he’s telling me to stay away from the net,’’ Djokovic said. “No I’m kidding. He’s a legend of the sport and glad to have him around. He’s been through these situations before, played here in this humidity. That’s where the advice comes in.’’